A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). The substrate may be supported on a substrate stage. Transfer of the pattern is typically during an exposure phase via imaging a patterned beam of radiation onto a layer of radiation-sensitive material (resist) provided on the substrate. A projection system is provided for projecting the patterned beam of radiation onto the target portion during the exposure phase. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers. In steppers each target portion is irradiated by exposing an entire pattern onto the target portion at one time. Other known lithographic apparatus include so-called scanners. In scanners each target portion is irradiated by scanning the pattern through a projection radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this given direction.
Before the exposure phase one or more properties of the substrate is/are sensed during a sensing phase. At least one of the one or more properties may be sensed by an alignment system. For example, a surface topography of the substrate may be measured during the sensing phase. This process is sometimes known as levelling or levelling scanning. Additionally or alternatively the property which is sensed may be the position of alignment marks on the substrate relative to other alignment marks provided on, e.g., the substrate stage on which the substrate is supported. This process is known as an alignment or alignment scanning. The properties sensed during the sensing phase are used during the exposure phase to ensure correct focus of the patterned beam of radiation on the substrate and/or correct positioning of the patterned beam of radiation on the substrate.
In one type of lithographic apparatus the substrate stage for supporting the substrate is in an environment with an ambient gas. Such a lithographic apparatus is called an atmospheric lithographic apparatus. In an atmospheric lithographic apparatus a liquid may be provided between the final element of the projection system and the substrate during the exposure phase. Such an apparatus is often referred to as an immersion lithographic apparatus.
The rate at which the lithographic apparatus applies the desired pattern on the substrate, known as throughput, is a major performance criterion in lithography apparatus. A higher throughput is desirable. Throughput is dependent on multiple factors. One factor on which throughput is dependent is the speed at which transfer of the pattern onto the substrate takes place. Another factor on which throughput is dependent is the speed at which properties of the substrate which need to be sensed prior to transfer of the pattern can be sensed. It is beneficial to have high moving speeds of the substrate during the exposure phase and/or during the sensing phase. However, it is important to maintain accuracy of measurements, particularly of determining the position of the substrate stage relative to the projection system, alignment system and/or alignment systems, at the high moving speeds.
Measurement radiation beams are used for determining the position of the substrate stage relative to the projection system, alignment system and/or alignment systems or an intermediate body such as a grid configured to cooperate with an encoder system or such as a reference frame. Measurement radiation beams in atmospheric lithographic apparatus pass through gas along a path of radiation. Local fluctuations in the characteristics of gas through which measurement radiation beams pass can affect the measurement radiation beam. Therefore, it is an aim of the present invention to provide an apparatus with reduced fluctuations in the characteristics of gas along the path of radiation.